1. Technical Field
This application relates generally to an antenna structure. More specifically, this application relates to an antenna that is responsive in at least two distinct frequency regimes whose resonators are coupled parasitically.
2. Background Information
Multiple frequency ranges have been allocated to handle the recent explosion of wireless communication devices and systems. Of the more recent devices, wireless communications devices such as laptop computers have been using the Bluetooth and 802.11 a/b frequency domains for wireless data transfer. Bluetooth, IEEE Standard 802.11 and the Japanese standard Hyperlan and their variants, are standards for wireless data communication. These standards are referred to collectively herein as 802.11a/b, although it will be recognized that some embodiments disclosed herein may be applied to other technologies as well. However, numerous problems exist with current antennas that must communicate in the 2.4 GHz and 5.2–5.8 GHz frequency domains specified by these standards.
One of these problems is the tradeoff between size and antenna efficiency: a relatively large size is necessary for a multi-frequency response antenna. Antenna performance must always be weighed against the size of the antenna. With any approach there will be a fundamental limit on the efficiency and bandwidth that can be achieved based on the total volume of the antenna. A smaller antenna is preferred for portable devices, such as laptop computers.
Traditionally, to gain more bandwidth in a particular band a matching network using lumped components is optimized, often in a pi or T network. However, with this solution, the achievable efficiency is limited to the realizable efficiency of the single element. Plus, the addition of lumped inductors and capacitors introduces loss.
Some of the best antenna solutions for 802.11a/b coverage in laptop computers presently are Planar Inverted F-Antennas (PIFAs). These narrow cross section antennas are designed to fit into very limited spaces around the display screen. However, PIFAs with very narrow cross sectional dimensions of 5 mm×5 mm or less have insufficient bandwidth to cover the 4.9 GHz to 5.85 GHz frequency range at a −10 dB return loss. To increase bandwidth to an acceptable range, the height or width of the PIFA must be increased beyond those permitted for installation near laptop computer displays.
A parasitic resonator has been used in conjunction with a PIFA to increase return loss bandwidth in handset antenna applications. This parasitic resonator is located above a ground plane and is coplanar with the PIFA. However, only the bandwidth of a single-band PIFA has been enhanced in this manner as typical handset applications. The single-band PIFA is both physically and electrically completely different from a PIFA that is designed to have a sufficient response in multiple frequency ranges. For example, if a lower frequency resonator is added, bandwidth is lost in the upper frequency range. Furthermore, emphasis in previous single-band PIFAs have been on a relatively wide and thin PIFA for handset form factors, which is incompatible with laptop computer use at least because of the stringent size requirements and thus design requirements in both. In addition, in the single-band PIFA with the parasitic resonator, the ground pin is located at an extremity of the antenna, i.e. the PIFA is fed conventionally.
Other 802.11b and/or Bluetooth antennas, which are also too large to fit next to laptop computer screens, include triband Bluetooth antennas for the 2.4/5.2/5.8 GHz bands from SkyCross, Inc., Melbourne, Fla., ranging in size from 20×18×3 mm to 22.3×14.9×6.2 mm. The smallest of these antennas appears to have an efficiency of better than 60% but has a poor Voltage Standing Wave Ratio (VSWR) of less than 3.0:1. The largest antenna is matched to better than a 2:1 VSWR but the efficiency is not listed (and is probably significantly lower due to the various tradeoffs involved in the design). Ethertronics, Inc., San Diego, Calif., offers a triband Bluetooth antenna that is only matched to −6 dB across the upper band (5.2–5.8 GHz) and has an estimated peak efficiency of 75% in the upper band (based on the return loss plot shown). Tyco Electronics Corporation, Wilmington, Del., also offers a circular triband Bluetooth Antenna with a diameter of 16 mm and a height of 6 mm. This antenna has a VSWR of better than 2.5:1 but like the larger SkyCross antenna has an unknown efficiency.
Thus, current multi-band antennas are not capable of meeting efficiency and overall compactness requirements for electronic devices, such as laptop computers, which use wireless communications in multiple frequency bands.